An undesired operating condition of a centrifugal separator of the kind stated above can arise from different causes and is often difficult to discover. This can lead to operating an impaired centrifugal separator for an extended period of time. One cause of an undesired operating condition is leakage through valves which were thought to be completely closed. For example, the centrifugal separator can be provided with a sludge discharge means, which intermittently discharges accumulated separated sludge together with liquid from the separation chamber with the aid of an axially moveable slide valve. Between the discharge movements the slide valve has to seal in order to cause the formation of a boundary layer between separated liquid phases at an intended radial level in the separation chamber. With respect to the function of the centrifugal separator it is important that the formed boundary layer is at the intended radial level. If liquid escapes out of the separation chamber via the slide valve the boundary layer is formed at a radial level which is situated outside the intended radial level, and results in a less than desirable separation. However, the centrifugal rotor is surrounded by a stationary protective cover making it impossible to visually identify a leaking slide valve.
SE-B-409 662 suggests a device for indicating leakage in a centrifugal separator that intermittently discharges sludge. The device comprises a vibration sensing piezoelectric crystal placed on the protective cover so that leaking liquid flow can contact the crystal. The concept is that the piezoelectric crystal is hit by droplets thrown from the rotating centrifugal rotor and generates a signal used to activate an alarm device.
For certain applications, for example the cleaning of oil from minor amounts of water and sludge, the centrifugal separator may be provided with three outlets for discharging three components from the separation chamber. In general, these outlets are a light phase outlet for a light component such as oil, a heavy phase outlet for a heavy component such as water and an outlet for a sludge component. If the amount of water separated is a small fraction of the amount of oil separated, the heavy phase or sludge outlet is provided with an intermittently opening valve for discharging the accumulated heavy phase (water) in batches. For the same reasons as described above in connection with the centrifugal separator with an intermittently opening slide valve for discharging sludge the heavy phase outlet valve has to be seal between openings in order to prevent the centrifugal separator from being operated at an undesired condition, or to prevent the loss of valuable product.
For other applications, for example the cleaning of liquid mixtures from large amounts of sludge, the centrifugal separator usually has a discharge means that continuously discharges separated sludge from the separation chamber by a plurality of nozzles. The nozzles are distributed around the centrifugal rotor at the radially outermost portion thereof. In this case an undesired operating condition of the centrifugal separator arises if the flow of sludge through the nozzles increases due to wear thereof, or if the sludge flow decreases due to clogging of one or more nozzles. Wear and clogging of the nozzles results in an impaired separation result.
An object of the present invention is to provide a reliable method of indicating an undesired operating condition in a centrifugal separator.